This invention relates to a gas generator for quickly supplying a large volume of gas to an inflatable bag type safety apparatus such as a gas bag for the protection of a car passenger in collision of the car, and more particularly to a gas generator of the type comprising a gas-generating composition confined in an apertured housing and a granular coolant which intervenes between the gas-generating composition and an apertured wall of the housing.
A recently developed automotive safety system includes an inflatable bag that momentarily and temporarily restrains a passenger and exhibits a cushioning effect during the critical instant of a collision impact.
A gas generator for use in such a system to quickly inflate the bag in case of need is desired to fully satisfy all of the following requirements:
(1) to be able to quickly produce a sufficiently large volume of gas upon initiation; PA1 (2) to be small in size so as to be installed in a narrow space, for example in a steering column, and light in weight so as not to hamper the ease of steering; PA1 (3) to be assuredly free from the possibility of explosion during operation or accidental firing by the influence of shocks; PA1 (4) to discharge a gas of which temperature is not so high as will cause the passenger to suffer burns upon contact with the inflated bag; PA1 (5) to discharge a non-toxic gas; and PA1 (6) to be stable over a long period of time, that is, to undergo no or little changes with the passage of time.
Considering the degrees of satisfactoriness in the important items such as quickness of gas generation, smallness in size and lightness in weight and safeness from explosion or accidental actuation, a gas generator of the type utilizing a solid gas-generating composition is advantageous over an another type of gas generator that is a pressure vessel confining therein either a pressurized gas or a liquefied gas as a component of an automotive gas bag system.
However, the use of a solid-gas-generating composition makes it necessary to take a certain measure for lowering the gas temperature since such a composition produces a gas by an exothermic decomposition reaction or combustion which proceeds at a very high rate so that the produced gas has an undesirably high temperature. Besides, sometimes the gas contains certain amounts of toxic reaction products and/or solid matter that may damage the inflatable bag. Therefore, it is also necessary to remove the toxic reaction products and the harmful solid matter from the generated gas.
To meet these requirements, gas generators comprising at least one cooling and filtering layer have been proposed in a variety of designs. Typical examples of the hitherto proposed techniques are as follows.
Japanese Patent Application, Primary Publication No. 49(1974)-13837 shows a gas generator comprising a cooling and filtering layer which is made of a wire screen or entwined fine wire of a metal having a high heat conductivity as exemplified by iron or steel, aluminum or copper. A disadvantage of this method is the necessity of using the wire screen or entwined wire in a large weight because every metal is relatively small in specific heat, resulting in an inevitable enlargement of the overall volume of the device. Moreover, the total weight of the device also increases because of the provision of a cooling layer of a metal, a material relatively large in specific gravity.
Then it has been proposed to use a certain compound which is endothermically decomposable and has a smaller specific gravity than metals, for example sodium hydrogen carbonate or ammonium carbonate, as a coolant in a gas generator by, for example, U.S. Pat. Nos. 3,305,319 and 3,515,518 and Japanese Patent Application, Primary Publication No. 50(1975)-146039. However, endothermically decomposable compounds are generally unsatisfactory in the rate of decomposition reaction and/or stability during long time storage.
Therefore, it is more popular to utilize a more stable heat-absorbing material such as alumina or silica in the form of fine granules as described in, for example, U.S. Pat. No. 3,785,674 and Japanese Patent Application, Primary Publication No. 52(1977)-130492. However, also this method is unsatisfactory in that the heat-absorbing material is relatively low in the cooling effect and hence needs to be used in a large weight and in the form of very fine granules to have a sufficiently large surface area relative to the weight and that the housing of the gas generator needs to have an augmented pressure resistance with an inevitable increase in weight because the employment of close packing of the fine granules of the heat-absorbing material to enhance the cooling effect offers a significant resistance to the flow of the gas generated in the gas generator housing.
Japanese Patent Application, Primary Publication No. 50(1975)-48797 discloses a gas generator comprising a porous and platy or tubular member made of a heat-absorbing material selected from a group of metal oxides including alumina and silica and a group of metal carbides including silicon carbide. This method has advantages such as enhanced cooling and filtering effects, uniformity and stableness of the enhanced effects and ease of assemblage of the device but involves a problem that the porous member offers a significant resistance to the gas flow.
In general, the temperature of a definite quantity of gas supplied from a gas generator containing a definite weight of gas-generating composition to an inflatable bag, assuming that the bag has a definite volume, is proportional to the pressure in the inflated bag according to the Boyle-Charles' law.
If the pressure in the inflated bag is not sufficiently high the bag may be squashed by the passenger subjected to a collision impact with the danger of the passenger colliding against a rigid member at the bottom of the bag. On the other hand, if the bag pressure is excessively high the bag will not serve an adequate cushioning effect but will impetuously repulse the passenger. The pressure in the inflated bag, therefore, must be within an appropriately predetermined range and, hence, the gas temperature must be within a predetermined range. When it is sure that the gas temperature does not exceed the upper boundary of the predetermined range, meaning that the gas generator contains a coolant which is high in cooling efficiency and always exhibits an expected cooling effect, it becomes unnecessary to add a considerable margin to the calculated quantity of the coolant so that it becomes possible to reduce the total weight of the gas generator. From another viewpoint, in case that a granular coolant inherently very high in cooling effect can be employed the mean particle size of the coolant granules can be made larger than the extremely small particle sizes required of less effective coolants such that the granules in a packed state offer less resistance to the gas flow in the gas generator. This results in lowering of the gas pressure in the gas generator, so that the housing of this device is allowed to have a lessened pressure resistance and therefore can be made lighter in weight.